Electrophotographic image forming apparatuses such as copiers and printers form visible images using one or more developing devices. Among various developing devices, non-contact developing devices which develop an electrostatic image formed on an image bearer such as a photoreceptor with a toner born on a toner bearer without contacting the toner bearer with the photoreceptor attract attention. Specific examples of these non-contact developing devices include powder cloud developing devices, jumping developing devices, and field curtain developing devices.
In jumping developing devices, a voltage is applied to a toner bearer to allow toner particles, which are adhered to the toner bearer at an adhesive force, to jump to the surface of an image bearer such as a photoreceptor.
In field curtain developing devices, an alternate electric field is applied to multiple electrodes arranged on a surface of a toner bearer at regular intervals to form an alternate unequal electric field, thereby forming a field curtain, so that previously charged toner particles on the surface of the toner bearer are supplied to an electrostatic latent image on a surface of an image bearer by hopping. In this regard, since toner particles hop from the surface of the toner bearer, and the adhesive force of the toner particles becomes substantially zero, it is not necessary to apply a force to the toner particles to release the toner particles from the surface of the toner bearer. Accordingly, in field curtain developing devices, toner particles can be fed to an image bearer at a relatively low applied voltage.
A field curtain developing device is proposed which uses a toner bearer in which an insulating protective layer is formed on the surface of multiple electrodes to prevent leaking of charges of toner particles to the electrodes, thereby preventing occurrence of defective hopping of the toner particles due to loss of charges of the toner particles.
Another field curtain developing device is proposed which has a toner bearer whose surface is made of a material capable of frictionally charging toner particles so as to have charges with a predetermined polarity so that toner particles supplied to the toner bearer hop and are desirably charged by applying an alternate voltage thereto.
In these field curtain developing devices, two kinds of electrodes having different electric potentials are arranged on an insulating layer. When the gap between the two kinds of electrodes is relatively narrow, electric leakage tends to occur therebetween. In this case, the electrodes cannot maintain the electric potentials, and therefore toner particles thereon cannot hop.
In attempting to prevent the leaking problem, a technique is proposed in which an insulating layer is formed on an electroconductive substrate serving as an electrode, and another electrode is formed on the insulating layer so that the two kinds electrodes are separated by the insulating layer.
In this case, when an alternate electric field or a rectangular pulse is applied to the electrodes to form an electric field allowing toner particles to hop, an electrostatic force is formed between the electroconductive substrate and the electrode formed on the insulating layer, thereby storing a large amount of charge. Therefore, a large amount of electric power is consumed, and it is necessary to reduce electric power consumption.
For these reasons, the inventors recognized that there is a need for a field curtain developing device in which a toner cloud is stably formed at a low electric power consumption so that toner particles are stably supplied to an electrostatic latent image on an image bearer.